Pyrometallurgical reactor cooling element and its manufacture

ABSTRACT

The invention relates to a method of fabricating a pyrometallurgical reactor cooling element with flow channels. In order to enhance heat transfer capability, the wall surface area of the flow channel, which is traditionally round in cross-section, is increased without increasing the diameter or length of the channel.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a coolingelement with flow channels for pyrometallurgical reactors. In order toenhance the heat transfer capability of the element, the surface area ofthe flow channel wall, which is traditionally round in cross-section, isincreased without increasing the diameter or length of the flow channel.The invention also relates to the element manufactured by this method.

BACKGROUND OF THE INVENTION

The refractory of reactors in pyrometallurgical processes is protectedby water-cooled cooling elements so that, as a result of cooling, theheat coming to the refractory surface is transferred via the coolingelement to water, whereby the wear on the lining is significantlyreduced compared with a reactor which is not cooled. Reduced wear iscaused by the effect of cooling, which brings about forming of so calledautogenic lining, which fixes to the surface of the heat resistantlining and which is formed from slag and other substances precipitatedfrom the molten phases.

Conventionally cooling elements are manufactured in two ways: primarily,elements can be manufactured by sand casting, where cooling pipes madeof a highly thermal conductive material such as copper are set in asand-formed mould, and are cooled with air or water during the castingaround the pipes. The element cast around the pipes is also of highlythermal conductive material, preferably copper. This kind ofmanufacturing method is described in e.g. GB patent no. 1386645. Oneproblem with this method is the uneven attachment of the piping actingas cooling channel to the cast material surrounding it. Some of thepipes may be completely free of the element cast around it and part ofthe pipe may be completely melted and thus fused with the element. If nometallic bond is formed between the cooling pipe and the rest of thecast element around it, heat transfer will not be efficient. Again ifthe piping melts completely, that will prevent the flow of coolingwater. The casting properties of the cast material can be improved, forexample, by mixing phosphorus with the copper to improve the metallicbond formed between the piping and the cast material, but in that case,the heat transfer properties (thermal conductivity) of the copper aresignificantly weakened by even a small addition. One advantage of thismethod worth mentioning is the comparatively low manufacturing cost andindependence from dimensions.

Another method of manufacture is used, whereby glass tubing in the shapeof a channel is set into the cooling element mould, which is brokenafter casting to form a channel inside the element.

U.S. Pat. No. 4,382,585 describes another much used method ofmanufacturing cooling elements, according to which the element ismanufactured for example from rolled or forged copper plate by machiningthe necessary channels into it. The advantage of an element manufacturedthis way, is its dense, strong structure and good heat transfer from theelement to a cooling medium such as water. Its disadvantages aredimensional limitations (size) and high cost.

The ability of a cooling element to receive heat can be presented bymeans of the following formula:

Q=α×A×ΔT, where

Q=amount of heat being transferred [W]

α=heat transfer coefficient between flow channel wall and water [W/Km²]

A=heat transfer surface area [m²]

ΔT=difference in temperature between flow channel wall and water [K]

Heat transfer coefficient a can be determined theoretically from theformula

 Nu=αD/λ

λ=thermal conductivity of water [W/mK]

D =hydraulic diameter [m]

Or Nu=0.023×Re{circumflex over ( )}0.8Pr{circumflex over ( )}0.4,

where

Re=wDρlη

w=speed [m/s]

D=hydraulic diameter of channel [m]

ρ=density of water [kg/m³]

η=dynamic viscosity

Pr=Prandtl number [ ]

Thus, according to the above, it is possible to influence the amount ofheat transferred in a cooling element by influencing the difference intemperature, the heat transfer coefficient or the heat transfer surfacearea.

The difference in temperature between the wall and the tube is limitedby the fact that water boils at 100° C., when the heat transferproperties at normal pressure become significantly worse due to boiling.In practice, it is more advantageous to operate at the lowest possibleflow channel wall temperature.

The heat transfer coefficient can be influenced largely by changing theflow speed, i.e. by affecting the Reynolds number. This is limitedhowever by the increased loss in pressure in the tubing as the flow rateincreases, which raises the costs of pumping the cooling water and pumpinvestment costs also grow considerably after a certain limit isexceeded.

In a conventional solution, the heat transfer surface area can beinfluenced either by increasing the diameter of the cooling channeland/or its length.

The cooling channel diameter cannot be increased unrestrictedly in sucha way as to be still economically viable, since an increase in channeldiameter increases the amount of water required to achieve a certainflow rate and furthermore, the energy requirement for pumping. On the,other hand, the channel diameter is limited by the physical size of thecooling element, which for reasons of minimizing investment costs, ispreferably made as small and light as possible. Another limitation onlength is the physical size of the cooling element itself, i.e. thequantity of cooling channel that will fit in a given area.

SUMMARY OF THE INVENTION

The present invention relates to a method of manufacturing a coolingelement for a pyrometallurgical reactor from a highly thermal conductivemetal such as copper, in which the heat transfer capability of saidcooling element is enhanced significantly by increasing heat transfersurface area so that it is economically feasible to manufacture athinner cooling element. This is done so that the wall surface area ofthe flow channel is increased without increasing the diameter of thecooling channel or adding length. The surface of the flow channel in thecooling element, which is essentially round in cross-section, isenlarged by forming grooves or threads on the inner surface of thechannel, by means of subsequent machining. As a result, a smallertemperature difference is required between the water and the coolingchannel wall with the same amount of heat, and furthermore, a lowercooling element temperature. The invention also relates to the coolingelement manufactured by this method. The essential features will becomeapparent in the attached patent claims.

In the cooling element described in the present invention, the heattransfer surface area is increased so that, although the cooling elementflow channel is basically round in cross-section, its wall is notsmooth, but by changing the contour of the wall very slightly, a greaterheat transfer surface area can be achieved with the same flowcross-sectional area (the same rate can be achieved with the same amountof water) compared with the unit of length of the cooling channel. Thisincrease in surface area can be achieved in the following ways:

A cooling element, manufactured by working, e.g. by rolling or forging,into which at least one flow channel which is round in cross-section ismachined for example by drilling, threads are machined afterwards on theinner surface of the flow channel. The cross-section of the channelremains essentially round.

A cooling element, manufactured by working, into which at least one flowchannel, which is round in cross-section is machined, rifle-like groovesare machined afterwards on the inner surface of the flow channel. Thecross-section of the channel remains essentially round.

Rifle-like grooves can be obtained advantageously by using a so-calledexpanding mandrel, which is drawn through the flow channel. The groovingcan be made for instance to a hole, closed at one end, in which case themandrel is pulled outwards. A hole can be made into a channel, which isopen at both ends either, by pushing or drawing a purpose-designed toolthrough the channel.

It is evident in all the methods described above that, if there aretransverse channel parts in the flow channel, seen from the castingdirection, these parts are made mechanically by machining e.g. drilling,and the holes which do not belong to the channel are plugged. Thebenefit of the method described in this invention was compared withprior art by using the attached example.

BRIEF DESCRIPTION OF THE DRAWINGS

With the example are some diagrams to illustrate the invention, wherein

FIG. 1 shows a principle drawing of the cooling element used in thetests,

FIG. 2 shows a cross-sectional profile of the test cooling element,

FIGS. 3a-3 d indicate the temperature inside the element at differentmeasuring points as a function of melt temperature,

FIG. 4 presents the heat transfer coefficient calculated from themeasurements taken as a function of the melt, and

FIG. 5 presents the differences in temperature of the cooling water andthe channel wall at different cooling levels for normalized coolingelements.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE

The cooling elements relating to the present invention were tested inpractical tests, where the bottom of said elements A,B,C and D wereimmersed in about 1 cm deep molten lead. Cooling element A had aconventional smooth-surfaced flow channel, and this element was used forcomparative measurements. The amount of cooling water and thetemperatures both before feeding the water into the cooling element andafterwards were carefully measured in the tests. The temperature of themolten lead and the temperatures inside the cooling element itself werealso carefully measured at seven different measuring points.

FIG. 1 shows the cooling element 1 used in the tests, and the flowchannel 2 inside it. The dimensions of the cooling element were asfollows: height 300 mm, width 400 mm and thickness 75 mm. The coolingtube or flow channel was situated inside the element as in FIG. 1, so,that the centre of the horizontal part of the tube in the figure was 87mm from the bottom of the element and each vertical piece was 50 mm fromthe edge of the plate. The horizontal part of the tube is made bydrilling, and one end of the horizontal opening is plugged (not shown indetail). FIG. 1 also shows the location of temperature measuring pointsT1-T7. FIG. 2 presents the surface shape of the cooling channels andTable 1 contains the dimensions of the test cooling element channels andthe calculatory heat transfer surfaces per meter as well as the relativeheat transfer surfaces.

TABLE 1 Flow cross-sectional Heat transfer Relative heat Diameter areasurface/1 m transfer surface mm mm² m²/1 m area A 21.0 346 0.066 1.00 B23.0 415 0.095 1.44 C 23.0 484 0.127 1.92 D 20.5 485 0.144 2.18

FIGS. 3a-3 d demonstrate that the temperatures of cooling elements B, Cand D were lower at all cooling water flow rates than the referencemeasurements taken from cooling element A. However, since the flowcross-sections of the said test pieces had to be made with differentdimensions for technical manufacturing reasons, the efficiency of theheat transfer cannot be compared directly from the results in FIGS. 3a-3d. Therefore the test results were normalised as follows:

Stationary heat transfer between two points can be written:

Q=S×λ×(T ₁ -T ₂),

where

Q=amount of heat transferred between the points [W]

S=shape factor (dependent on the geometry) [m]

λ=thermal conductivity of the medium [W/mK]

T₁=temperature of point 1 [K]

T₂=temperature of point 2 [K]

Applying the above equation to the test results, the followingquantities are obtained:

Q=measured thermal power transferred to cooling water

λ=thermal conductivity of copper [W/mK]

T₁=temperature at bottom of element as calculated from tests [K]

T₂=temperature of water channel wall as calculated from tests [K]

S=shape factor for a finite cylinder buried in a semi-infinite medium(length L, diameter D) shape factor can be determined according to theequation

S=2πL/ln(4z/D)

when Z>1.5D,

z=depth of immersion measured from the centre line of the cylinder [m]

The heat transfer coefficients determined in the above way are presentedin FIG. 4. According to multivariate analysis a very good correlation isobtained between the heat transfer coefficient and the water flow rateas well as the amount of heat transferred to the water. The regressionequation heat transfer coefficients for each cooling element arepresented in Table 2.

Thus a [W/m²K]=c+a×v [m/s]+b×Q[kW].

TABLE 2 C A b r² A 4078.6 1478.1 110.1 0.99 B 3865.8 1287.2  91.6 0.99 C2448.9 1402.1 151.2 0.99 D 2056.5 2612.6 179.7 0.96

To make the results comparable, the cross-section areas of the flowchannels were normalized so that the amount of water flow corresponds tothe same flow rate. The flow channel dimensions and heat transfersurface areas normalized according to the flow amount and rate arepresented in Table 3. Using the dimensions given in Table 3 for casesA′, B′, C′ and D′ and the heat transfer coefficients determined asabove, the temperature difference of the wall and water for normalizedcases regarding the flow amount were calculated as a function of waterflow rate for 5, 10, 20 and 30 kW heat amounts with the equation

ΔT=Q/(α×A)

TABLE 3 Flow cross-sectional Heat transfer Relative heat Diameter areasurface/1 m transfer surface mm mm² m²/1 m area A* 21.0 346 0.066 1.00B* 21.0 346 0.087 1.32 C* 19.2 346 0.120 1.82 D* 15.7 346 0.129 1.95

The results are shown in FIG. 5. The figure shows that all the coolingelements manufactured according to this invention achieve a certainamount of heat transfer with a smaller temperature difference betweenthe water and the cooling channel wall, which illustrates theeffectiveness of the method. For example, at a cooling power of 30 kWand water flow rate of 3 m/s, the temperature difference between thewall and water in different cases is:

ΔT [K] Relative ΔT [%] A′ 38 100  B′ 33 85 C′ 22 58 D′ 24 61

When the results are compared with the heat transfer surfaces, it isfound that the temperature difference between the wall and the waterneeded to transfer the same amount of heat is inversely proportional tothe relative heat transfer surface. This means that the changes insurface area described in this invention can significantly influence theefficiency of heat transfer.

What is claimed is:
 1. A method for enhancing the heat transfercapability of a pyrometallurgical reactor cooling element with a coolingwater flow channel, fabricated of highly thermal conductive metal,comprising: forming the cooling element of a wrought copper plate;machining into the cooling element at least one cooling water flowchannel comprised of a plurality of straight channel parts, at least oneof said flow channel parts being transverse to another of said flowchannel parts, each of said flow channel parts being essentially roundin cross-section; and subsequently machining threads or rifle-likegrooves on the inner surface of each flow channel part to increase thewall surface area of the flow channel part inside the cooling elementwithout increasing the diameter or length of each flow channel.
 2. Themethod according to claim 1, wherein said cooling water flow channel ismachined to form a U shape, said U-shape being formed by three coolingwater flow channel parts, a first and a second of said cooling waterflow channel parts being machined so that they are substantiallyparallel to one another and a third of said cooling water flow channelparts being machined so that it is substantially transverse to the firstand second cooling water flow channel parts, said first, second andthird water flow channel parts being in fluid communication with oneanother.
 3. The method according to claim 2, further comprisinginserting a plug into an end of at least one of said first, second andthird cooling water flow channel parts to place the first, second andthird cooling water flow channel parts in fluid communication with oneanother and to form said cooling water flow channel.
 4. A method forenhancing the heat transfer capability of a pyrometallurgical reactorcooling element with a cooling water flow channel, fabricated of highlythermal conductive metal, comprising forming the cooling element of awrought copper plate; machining at least one cooling water flow channelthat is essentially round in cross-section into the cooling element; andsubsequently machining threads or rifle-like grooves on the innersurface of the flow channel to increase the wall surface area of theflow channel inside the cooling element without increasing the diameteror length of the flow channel.
 5. The method according to claims 4,wherein the rifle-like grooves are made by means of an expandingmandrel.